1. Field of the Invention
The present invention relates to an ink jet printing apparatus, and more specifically, to an ink jet printing apparatus that executes printing by scanning a printing head in two directions.
2. Description of the Related Art
With the recent spread of personal computers, word processors, facsimile machines, and the like to offices and homes, printing apparatuses based on various printing systems have been provided as information output equipment for the above equipment. In particular, printing apparatuses such as printers which are based on an ink jet system can be relatively easily adapted to execute color printing using plural types of inks. The ink jet printing apparatus has various advantages; for example, it makes only a low noise during operation, can achieve high grade printing on a variety of print media, and is small in size. In this respect, the printer based on this system and the like are suitable for personal use at office or home. Of these ink jet system-based printing apparatuses, a serial type in which a printing head reciprocates to perform printing to a printing medium is very popular because it is inexpensive and can print high grade images.
In spite of its relatively low costs, the serial type printing apparatus is desired to exhibit a higher performance. The printing performance is typified by image quality or image grade, and printing speed.
One of factors that determine image quality or the like is the type of ink. In general, the use of more or appropriate types of inks allows a higher-quality image to be printed. The inks can be classified into dye inks, pigment inks, and the like on the basis of coloring materials used for the inks, or dark and light inks on the basis of the concentration of the coloring materials, or a special color such as orange, red, blue inks, and the like on the basis of ink colors.
Well-known printers use, for example, six types of inks including a dye black ink, a dye yellow ink, a dark and light dye magenta inks, and a dark and light dye cyan inks, or four types of inks including a pigment black ink, a dye yellow ink, a dye magenta ink, and a dye cyan ink. The former apparatus focuses on the output to gloss printing media of photographic images of high quality inputted using a digital camera, a scanner, or the like. The latter apparatus focuses on the high-grade output to ordinary paper of black lines such as black letters and charts.
In general, to obtain a high optical reflection density for black, pigment coloring materials such as carbon black are used to perform printing to an ordinary paper rather than using dye color materials as described above. This is because the pigment is dispersed in the ink and because when this ink is applied to the ordinary paper, the dispersion becomes unstable to cause coagulation, resulting in the effective coverage of the surface of the printing medium. Further, when the ink has a surface tension of about 40 dyne/cm, this prevents the ink from bleeding along fibers in the ordinary paper. Such ink designs enable the printing of letters and lines having a high contrast with respect to the surface of the paper as well as sharp edges. On the other hand, the dye dissolves in the ink at a molecular level, whereas the pigment is dispersed in the ink and thus has relatively large coloring material grains. Thus, the pigment cannot pass through a gloss layer in the surface of a glossy printing medium. The pigment accumulates in the surface of the gloss layer to reduce the glossiness.
Thus, when performing printing to a gloss printing medium, the above printing apparatus using a pigment black ink often expresses a black component of an image by using what is called a process black composed of three color inks, a dye yellow ink, a dye magenta ink, and a dye cyan ink, instead of using a pigment black ink. However, to improve the contrast of a black image in a print, it is more preferable to use a dye black ink than to use the three-color inks. In this case, only the dye black ink is used, thus enabling a reduction in the amount of ink applied per unit area of a printing medium. This prevents problems such as ink bleeding. Further, if a gray level is to be expressed in a print image, dots for a color of a relatively high gray level are generally formed by applying a black ink as well as a cyan, magenta, and yellow inks.
In this manner, combinations of various inks are used depending on the type of images to be printed or printing media used. For example, when ordinary paper is important, the apparatus is configured to use a pigment black ink. If gloss printing media are important, the printing apparatus uses a dye black ink.
In contrast, Japanese Patent Application Laid-open No. 11-001647 (1999) describes a configuration focusing on both ordinary paper and gloss printing media. According to this document, the configuration has printing means for a pigment black ink and printing means for a dye black ink. It does not use the pigment black ink but only the dye black ink to perform printing to printing media that have a gloss layer and an ink receiving layer and that are incompatible with the pigment black ink. It uses the pigment black ink to perform printing to the ordinary paper. In this manner, this configuration can print a high-quality or -grade image on both ordinary paper and gloss print media.
Bidirectional printing is known as a configuration that can improve the printing speed, belonging to the printing performance. With this printing system, in a serial type printing apparatus, the printing head is first scanned in a forward direction for printing. Then, paper is fed by a predetermined amount, and printing scan is subsequently executed again by moving the printing head in a backward direction. This printing system achieves an approximately double printing speed or throughput compared to unidirectional printing in which printing is executed during forward scanning, whereas it is not executed while the printing head is moving in the backward direction. Other known printing systems include what is called one pass printing in which one scan completes printing of a scan area of a width equal to the arrangement width of ejection openings in the printing head, and what is called multi-pass printing in which printing is completed by a plurality of scans between which paper feeding is interposed. The above bidirectional printing system can also achieve the one pass printing and multi-pass printing. If the one pass printing is executed using the bidirectional printing system, the printing speed or throughput can be maximized.
The bidirectional printing system is effective means in improving the printing speed or the like as described above. However, this system is known to vary colors with scan areas, leading to non-uniform colors or color drifts in a printed image. This is because the application order of the color inks differs between the forward and backward directions of the bidirectional printing. In the printing apparatus, ejection opening rows for the respective color inks are commonly arranged in the scanning direction. However, in this case, the application order may be reversed between the forward scanning and the backward scanning depending on the arrangement of the ejection opening rows.
If dots of a predetermined color are to be formed by applying (ejecting) plural types of inks so that these inks are superposed on a pixel, inks applied to a printing medium earlier more favorably develop their colors. This is because the inks applied to the printing medium earlier easily color the material in a layer closer to the front surface of the printing medium, while the inks applied to the printing medium later less easily color the material in the front surface of the printing medium and permeates deeper through the printing medium in its thickness direction before they are settled. This phenomenon is significant if the ink receiving layer is composed of coat paper consisting of silica. However, it also occurs on ordinary paper or gloss printing media having a gloss layer formed in their front surface and an ink receiving layer formed inside the gloss layer.
Japanese Patent Application Laid-open Nos. 2000-318189 (for example, FIG. 6) and 2001-096771 (for example, FIG. 5) describe a configuration that can avoid non-uniform colors or the like attributed to the application order of inks. In this configuration, two nozzle rows are provided for the respective color inks and arranged symmetrically with respect to an axis orthogonal to the scanning direction. These documents disclose the configuration in which nozzle rows c1 and c2 for a cyan ink, nozzle rows m1 and m2 for a magenta ink, and nozzle rows y1 and y2 for a yellow ink are each arranged symmetrically with respect to a predetermined axis of symmetry orthogonal to the scanning direction of the printing head, for example. In this configuration, to form an ink dot for each pixel, the inks are ejected (applied) in order of c1, m1, y1, y2, m2, and c2 in the forward scanning direction. The inks are ejected (applied) in order of c2, m2, y2, y1, m1, and c1 in the backward scanning direction. This enables the inks to be applied or superposed on one another in the same order between the forward scanning and the backward scanning. Further, Japanese Patent Application Laid-open No. 2001-096771, mentioned above, discloses a configuration in which a symmetrical arrangement is also used for the black ink. However, Japanese Patent Application Laid-open Nos. 2000-318189 and 2001-096771, mentioned above, also disclose a printing head configuration with one or two nozzle rows in which the above symmetrical arrangement is not used for the black ink.
Specifically, these documents disclose a printing head that applies different types of inks to each pixel, the printing head being composed of nozzle rows that eject the cyan, magenta, and yellow inks, for which two types of application orders are available in relation to the scanning direction, and nozzle rows for the black ink, for which the application order varies depending on the scanning direction, in connection with the other types of inks.
Further, Japanese Patent Application Laid-open No. 2001-096771 uses the above printing head configuration and uses a mixture of pixels with different application orders in printing an image so that for the entire image, the number of pixels with one application order is substantially the same as that with another application order. This configuration reduces the non-uniformity of the colors attributed to the application order regardless of print data.
However, if the number of inks used in an apparatus capable of bidirectional printing is simply increased in order to consistently improve image quality and grade as well as the printing speed, the print head must be configured so that symmetrically arranged nozzle rows are also provided for these inks. If a dye black ink, a light magenta ink, a light cyan ink, or what is called a special color ink such as an orange ink, a red ink, or a green ink is used, two symmetrical nozzle rows must be provided for these inks. This results in an increase in the size of the printing head. Therefore, this configuration is disadvantageous in terms of costs and the size of the printing apparatus.
On the other hand, if asymmetrically arranged nozzle rows are additionally provided for the added inks, then the order in which this ink and the other inks are applied varies depending on the scanning direction. Then, the resultant colors may not be uniform, thus degrading the image quality.